1. Field of the Invention
The present embodiments relate to dry gas seals used in cryogenic liquid turbines and cryogenic liquid pumps.
2. Background Art
Dry gas seals are used in turbomachines (turbines and compressors) for shaft sealing purposes. A dry gas seal minimizes the amount of seal gas needed to isolate the gas being expanded or compressed from internal machinery components such as bearings, the latter usually containing oil or grease. This is achieved by a pair of non-contacting rings, i.e. one ring mounted on a rotating shaft and another ring mounted on a stationary component of the turbomachine. The two rings are separated by a low clearance gap through which the seal gas flows.
This type of sealing has been used in cryogenic vertical pumps to minimize the amount of seal gas (also called buffer gas) used. The seal gas may be the process gas itself, which is vaporized from the pumped cryogenic liquid. In certain cryogenic vertical pumps, dry gas seals are located at the top of the pump shaft, this location enabling the sealing components to remain in a gaseous atmosphere. Therefore, no vaporization is needed for the fluid flowing between the seal rings. However, in cryogenic horizontal pumps where the seal is located at the suction side of the pump, the sub-cooled liquid needs to be heated to a temperature near its boiling point to vaporize between faces of the respective seals.
The use of dry gas seals in a cryogenic liquid environment, without external buffer gas, requires that the sealing fluid is vaporized when flowing through the dry gas seal. The frictional heat absorbed by the fluid when flowing between non-contacting seal faces should be large enough to vaporize the liquid and create the gas film between the seal faces. A stable gas film is indeed necessary to minimize the seal clearance and therefore minimize the gas leakage flow through the seal. Fluid cooling from heat exchange with dry gas seal components may result in the liquid remaining too cold (and too far from its boiling point) to vaporize. As a result, erratic and excessive gas flow leakage may be observed in certain horizontal pumps, where the seal is in a direct cryogenic sub-cooled liquid environment. These erratic and excessive gas flows need to be eliminated.
For example, referring to FIGS. 1 and 2, a known cryogenic horizontal pump is shown generally at 10 which includes a suction side 12 and an exhaust side 14 of a casing 15 or housing of the pump. The pump 10 is used for moving or transferring cryogenic fluid, such as a cryogenic liquid, therethrough. The cryogenic fluid can be selected from liquid natural gas (LNG), liquid nitrogen (LIN), liquid oxygen (LOX), liquid argon (LAR), liquid CO2 and liquid air. At the suction side 12, a dry gas seal is provided such as that shown in FIG. 2. The dry gas seal includes a rotating ring portion 16 and a stationary ring portion 18 which coact to retain vaporized gas from the cryogenic liquid flowing through the pump 10.
Referring to FIG. 2, the sealing arrangement results in an excessive seal gas flow F, typically about 5-10 NI/min. The excessive seal gas flow is a result of the cryogenic liquid vaporizing between the seal portions to provide the dry gas for the seal, which is problematic in that it results in an excessive amount of the dry gas leaking from the seal. The known seal arrangement includes a pump shaft 20 about which a pump inlet chamber 22 is disposed for receiving cryogenic liquid. The cryogenic liquid is usually at −196° C. and 1 barg. The stationary ring portion 18 of the seal is mounted to the seal seat 24, and the rotating ring portion 16 of the seal is mounted to the pump shaft 29. The pump inlet chamber 22 provides the cryogenic liquid from which the evaporated gas for the dry gas seal is to be provided.
What is needed however is a gas seal assembly which provides the cryogenic liquid at a pressure and a temperature as close as possible to the liquid's boiling point (vapor phase) at the inlet to the gas seal rings.